Infections with the opportunistic mold Aspergillus fumigatus continue to be a major threat to the management of patients with hematologic malignancies and organ transplants. Since current antifungal therapies are unable to prevent a high rate of mortality for this disease, there is a need for more information on fungal pathways that allow this organism to thrive in the host environment. Recent studies have shown that A. fumigatus, and other pathogenic fungi, rely heavily on the unfolded protein response (UPR) for infection. The UPR is an adaptive response to endoplasmic reticulum (ER) stress, responsible for adjusting the protein folding capacity of the ER in proportion to the demand placed on the secretory pathway. In mammals, an important branch of the UPR is translational repression mediated by eIF2 phosphorylation, a pathway that lowers the influx of nascent proteins into the ER when the organelle is overloaded. Our preliminary data demonstrate that an analogous pathway is present in A. fumigatus, which challenges the existing paradigm of fungal UPR signaling. The overarching hypothesis to be tested in this proposal is that translational repression is a vital stress response of A. fumigatus that contributes to the virulence and antifungal drug susceptibility of this organism. The first aim will use mutants of the pathway to determine the impact of eIF2 phosphorylation and translational repression on the ability of the fungus to cause infection and to withstand antifungal drugs and other types of stress. Secondly, an important characteristic of the mammalian pathway is that a limited subset of mRNAs with key functions in ER homeostasis can escape translational repression. Thus, a second aim will use RNA-seq and polysome fractionation to identify mRNAs that show increased translation during ER stress. The outcome of this study is expected to uncover a new paradigm for the fungal ER stress response, which could lead to the development of novel strategies to combat infections with A. fumigatus, and potentially other eukaryotic pathogens that exploit the UPR for virulence.